Blood vessels have an endothelial cell layer covering the inside surface of the blood vessel. The blood flowing through the vessel interacts with the endothelial cell (EC) layer. Blood flows at different rates through different sized vessels, and is based on an equilibrium response between the shear stress exerted on the EC layer and the vessel diameter. The vessel prefers a given shear stress, thus larger vessels reflect a lower shear stress from slower blood flow, and smaller vessels reflect the higher shear rates from faster flow. Mechanical stress also stimulates a blood vessel tissue response, with increased tissue stress (trauma) producing a thickening of the vessel wall tissue, referred to as a hyperplastic response. When vessels are repaired or replaced, the vessel ends or graft and vessel ends are attached or sewn together. This seam is called an anastomosis and it is the region of connection between blood vessels or sections of blood vessels and generally, sutures are used to create an anastomosis.
When larger diameter (above about 6 mm) blood vessels are sutured together or engaged with a woven graft to create an anastomosis, the presence of the suture creates a circumferential ring around the connected vessel anastomosis that is five to seven times stiffer than the adjacent vessel tissue (Trubel, et al, ASAIO Journal, 1994; J. Vasc. Surg., 10, 1996). This is because available sutures utilized for vessel anastomoses are relatively stiff (high axial modulus) compared to the tissue having a low axial modulus. Although current sutures can easily bend and loop, the resistance to stretch in the long direction of the suture is very high. That is, the axial modulus is very high and more than 100 times that of the modulus of typical blood vessel tissue. Such stiff suture materials include Dacron, silk, PTFE, prolene, nylon, and polyalycolic/polylactic acid resorbable sutures.
When blood under pressure is returned to the sutured vessels, the current stiff circumferential suture ring keeps the anastomosis site from dilating in a manner similar to that of the adjacent venous or arterial vessels. When current stiff prolene and other stiff sutures are used, the stiffer anastomosis resists dilation and creates a flow discontinuity as well as a flow restriction along the inside diameter of the blood vessel. This discontinuity can be as great as one millimeter (Y. Kim and K. Chandran, Biorheology, 30, 117-130, 1993). While this "ridge" along the inside diameter of the vessel may be tolerable for larger diameter vessel, it creates a strong tendency for smaller-diameter vessels to thicken (hyperplasia). It also produces high and low shear regions as blood flows over the ridge.
The presence of this ridge in a coronary bypass graft anastomosis can also account for the incidences of short-term thrombosis and occlusion as well as the longer-term occlusion resulting from vessel hyperplasia and still longer-term atherosclerosis. This happens because on the downstream side of the ridge, a low shear region occurs. The low-shear stimulates the vessel to thicken which reduces its diameter and increases shear stress. This reduces the amount of allowable blood flow in the vessel. The downstream side of the ridge develops a flow eddy and low shear stress which then can form a thrombogenic process leading to occlusion (blockage) of the vessel or release of emboli (detached clots).
The presence of the discontinuity (ridge) in the vessel wall produces an increased longitudinal wall stress at the site of anastomosis (suturing), which can produce hyperplasia. In the smaller diameter coronary blood vessels, a one millimeter ridge cannot be tolerated. The vessels are already less than 4 mm, and a 1 mm ridge, or even a 1/2-mm ridge, presents a dramatic and severe restriction to flow and a hyperplastic response from the stress concentration at the anastomosis site. It is this situation that helps to explain the poor success of woven polymer fiber vascular grafts for vessels less than about 6 mm diameter (MDI, Inc., 1996, Chap. 5), and the unsatisfactory 80 to 90 percent patency rate of coronary artery bypass grafts within one year of a bypass procedure.
Another longer term result can be the constant stress concentration from current stiff sutures within the vessel tissue which, over time, can contribute to atherosclerosis of the sutured vessel. With these results occurring with currently utilized stiff silk, prolene, nylon, and other type suture materials, it is not surprising that the anastomosed bypass vessels lose patency rapidly after surgery. One study showed these sutured vessels to have an 80 percent patency rate at one year, decreasing to as low as 20 percent at five years post-operatively (Lei, et al, J. Biomechanics, 29, 1996, p 1605).
U.S. Pat. No. 4,550,730 to Shalaby and Schipper describes the use of copolymers of polymethylene terephthalate that are drawn with a tensile strength greater than 60,000 psi and a modulus between 80,000 and 280,000 psi. The tensile elongation is specified between 20 and 80 percent. In contrast, the present invention describes a polymeric suture material or construct with an elastic modulus less than about 2000 psi and with a tensile elongation above about 100 percent. Additionally, the U.S. Pat. No. 4,550,730 does not describe the use or benefit of a low modulus suture of properties described in the present invention for blood vessel anastomoses.
U.S. Pat. No. 4,543,952 to Shalaby and Koelmel describes a similar suture material to that described in the U.S. Pat. No. 4,550,730, but made of polyester copolymers. Although the elastic modulus is less than 250,000 psi, the required tensile strength is above 45,000 psi. Additionally, the U.S. Pat. No. 4,543,952 does not describe or teach the use or benefit of an isoelastic suture for blood vessel anastomoses.
There are earlier patents referenced in the Shalaby patents which describe a low compliance polymer material. An example is U.S. Pat. No. 3,454,011 which proposed the use of a spandex polyurethane. However, this was proposed as a general suture material and did not specify the desirability or use of such a material for blood vessel anastomosis, as in the present invention. Similarly, U.S. Pat. No. 3,954,689 to Hoeschele et al. described a rubbery film material made of a polybutylene terephthalate thermoplastic. However, a filamentous use of this material for suturing blood vessels is not described. U.S. Pat. No. 5,620,702 also describes low-modulus films and sutures, but the construct is a laminated device with an adhesive layer and does not specify the use of a low-modulus core polymer filament with modulus less than about 2000 psi, as does the present invention. Further, the use or desirability of such a low-modulus polymer suture for improving blood vessel anastomoses is not described.
It is an object of the present invention to use polymeric suture materials that possess inherent low modulus (high stretchability, or compliance) or are constructed to have a low modulus under tension to a particular displacement. Alternatively, a higher stiffness (modulus) polymeric suture material and device can be used which is then modified in place (in-situ) via various types of energy to reduce the modulus while in place. Such thermal energy can include light, radiation, heat, vibration or a chemically activated reduction in modulus.
The inventive material or device is described as isoelastic, which means that its elastic properties closely approximate the elastic (stiffness) properties of the vessel tissue being sutured. Elastic sutures for other wound closure applications have been proposed, but with modulus of elasticity between the 500 to 2000 psi preference of the present invention. Further, a high tensile elongation (above 100 percent) as described in the present invention is not disclosed in the prior art. Specifically, there is no mention of the use of silicone, polyurethanes, its copolymers, or other thermoplastic elastomers for vessel suture applications, and which also meet the stiffness and elongation requirements of the present invention. There are also no prior art references that describe the in-situ modification of stiffer polymers via light, heat, radiation or other energy or chemically activated to reduce the modulus after suturing.